net: ipv4: Consolidate ipv4_mtu and ip_dst_mtu_maybe_forward

Consolidate IPv4 MTU code the same way it is done in IPv6 to have code
aligned in both address families
Signed-off-by: Vadim Fedorenko <vfedorenko@novek.ru>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip.h

@@ -436,18 +436,32 @@ static inline bool ip_sk_ignore_df(const struct sock *sk)
 static inline unsigned int ip_dst_mtu_maybe_forward(const struct dst_entry *dst,
 						    bool forwarding)
 {
+	const struct rtable *rt = container_of(dst, struct rtable, dst);
 	struct net *net = dev_net(dst->dev);
 	unsigned int mtu;
 
 	if (net->ipv4.sysctl_ip_fwd_use_pmtu ||
 	    ip_mtu_locked(dst) ||
-	    !forwarding)
-		return dst_mtu(dst);
+	    !forwarding) {
+		mtu = rt->rt_pmtu;
+		if (mtu && time_before(jiffies, rt->dst.expires))
+			goto out;
+	}
 
 	/* 'forwarding = true' case should always honour route mtu */
 	mtu = dst_metric_raw(dst, RTAX_MTU);
-	if (!mtu)
-		mtu = min(READ_ONCE(dst->dev->mtu), IP_MAX_MTU);
+	if (mtu)
+		goto out;
+
+	mtu = READ_ONCE(dst->dev->mtu);
+
+	if (unlikely(ip_mtu_locked(dst))) {
+		if (rt->rt_uses_gateway && mtu > 576)
+			mtu = 576;
+	}
+
+out:
+	mtu = min_t(unsigned int, mtu, IP_MAX_MTU);
 
 	return mtu - lwtunnel_headroom(dst->lwtstate, mtu);
 }

net/ipv4/route.c

@@ -1299,26 +1299,7 @@ static unsigned int ipv4_default_advmss(const struct dst_entry *dst)
 
 INDIRECT_CALLABLE_SCOPE unsigned int ipv4_mtu(const struct dst_entry *dst)
 {
-	const struct rtable *rt = (const struct rtable *)dst;
-	unsigned int mtu = rt->rt_pmtu;
-
-	if (!mtu || time_after_eq(jiffies, rt->dst.expires))
-		mtu = dst_metric_raw(dst, RTAX_MTU);
-
-	if (mtu)
-		goto out;
-
-	mtu = READ_ONCE(dst->dev->mtu);
-
-	if (unlikely(ip_mtu_locked(dst))) {
-		if (rt->rt_uses_gateway && mtu > 576)
-			mtu = 576;
-	}
-
-out:
-	mtu = min_t(unsigned int, mtu, IP_MAX_MTU);
-
-	return mtu - lwtunnel_headroom(dst->lwtstate, mtu);
+	return ip_dst_mtu_maybe_forward(dst, false);
 }
 EXPORT_INDIRECT_CALLABLE(ipv4_mtu);